Just press Cmd + R while rebooting, and you will be given a special screen where you can choose "Get help online" which is basically Safari mode.
It's not very useful though as it requires completely rebooting before and after using Safari mode, and things like the trackpad tap-to-click doesn't work, and none of your personal settings work. It's good if your system crashes and you need to read the forums about how to fix it, but you won't be using it as a sort of "Guest Account" for your wife or whatever, it's just too clumsy to reboot twice, especially that Lion takes ages to reboot.
It will be cool for students to be able to browse the net even without a password, which will annoy the hell out of their tutors, so I guess they will quickly revert back to Snow Leopard as they realize this.
